This invention relates to a power aggregate or engine having a casing with at least one internal wall which, during operation of the aggregate or engine, is thermally stressed and subjected to vibration, and in which the aggregate or engine has at least one space between such internal wall and an outer wall of the casing in which a liquid coolant is circulated to cool the internal wall. Typically, water or oil is used as a coolant in aggregates or engines of this character. In practice, a problem is that the coolant in the aforesaid space is practically incompressible and carries the vibrations developed during operation of the aggregate or engine directly to the outer walls of the engine casing, thereby producing considerable noise emission from the engine or aggregate. This phenomenon is particularly noticeable in internal combustion engines in the regions of the cylinder which encloses the combustion chamber and the cylinder head which contains the valves and the inlet and exhaust ducts, and leads to a substantial production of noise in the vicinity of the engine.